This invention concerns hearing devices (particularly including hearing aids) and their assembly, and is especially concerned with the long-felt need to minimize or avoid the unwanted cascading amplification (i.e., feedback) caused by vibrations of either the casing or the components of hearing devices, particularly including the hearing aid.
Hearing aids, particularly in-the-ear and in-the-canal aids, have become exceedingly small. The casing of such a hearing aid usually contains electrical components including a microphone, an amplifier, and a loudspeaker assembly (usually called a xe2x80x9creceiverxe2x80x9d), which, because of their tiny size, are both delicate and difficult to handle. The close proximity of the microphone and receiver in the casing makes it difficult to avoid acoustic feedback.
U.S. Pat. No. 4,969,534, incorporated herein by reference, describes a hearing aid that employs a viscoelastic material to adhere components to the casing of the hearing aid, describes other prior art attempts to reduce amplification of noise by a hearing aid and to facilitate their assembly, and describes and identifies other prior art.
A currently used method for making a custom in-the-canal hearing aid for a person includes pouring pre-polymerized silicone rubber into the ear canal of the person and allowing it to solidify into a molding having the same outer shape as the inner shape of that ear canal. Agar or a different silicone rubber is poured around that molding after it is removed from the ear canal to make a receptacle with a cavity having an inner surface of the same shape as the person""s ear canal. After removing the molding from the cavity, a layer of methyl methacrylate pre-polymer resin is poured along the inner surface defining that cavity. That layer is solidified to provide a portion of a casing for the custom hearing aid that has a custom irregular outer surface shaped to be closely received in that person""s ear canal, has an irregular inner surface generally corresponding to its outer surface that defines a cavity, and has an access opening to that cavity. A through opening is formed between the inner and outer surfaces of the casing in a portion of the casing that will be received innermost in the ear canal. A sound receiving, amplification and transmitting system is mounted within the cavity in the casing. That system includes components mounted on a faceplate of a material that is compatible with the material of the casing, which faceplate is adapted to be permanently attached across the access opening to the cavity in the casing. Those components on the faceplate are attached by electrical connections to a loudspeaker assembly called a receiver having a sound outlet opening in a first end of the receiver. A tube of resiliently flexible material having a through passageway between opposite first and second ends has a portion adjacent its first end engaged around the receiver from its first end to a second opposite end of the receiver with the sound outlet opening of the receiver communicating with the passageway, and has the second end of the tube positioned through opening in the casing. The faceplate is permanently attached across the access opening to the cavity, leaving the receiver supported in the cavity by the tube and the electrical connections from the components of the system mounted on the faceplate. The system is then activated, while a technician listens to the system with a stethoscope at the second end of the tube and determines from the quality of sound emanating from that tube whether the receiver is supported without significant contact with the casing or other components in the cavity, for if it is not a significant amount of amplified noise and feedback will be heard. If such is the case, the technician will relocate the receiver in the cavity by manipulating the tube (e.g., by pulling or pushing on the tube and/or rotating it about its axis) until a position for the receiver is found where it will not have significant contact with the casing or other components in the cavity, whereupon the second end of the tube is secured to the casing to maintain that position. This adjustment can take a significant amount of time and can significantly add to the cost of producing the custom hearing aid. Such support for the receiver isolates its vibrations from the casing and other components in the cavity, but provides little damping of the vibrations from the receiver.
The present invention provides a method for mounting a transducer particularly including a receiver in a hearing device such as a hearing aid (particularly including a custom in-the-ear hearing aid generally of the type described above) using a special support pad that can provide placement of and support for the receiver that will restrict the amount of feedback that will be produced without the need to position the receiver in the casing using the technique described above.
The support pad according to the present invention can be used in a hearing aid (e.g., a custom in-the-ear hearing aid) of the type comprising (1) a molded casing having an irregular outer surface shaped to be received in a person""s ear canal, having an inner surface defining a cavity, and having a through opening between its inner and outer surfaces in a portion of the casing to be received innermost in the ear canal; (2) a sound receiving, amplification and transmitting system within the cavity in the casing, which system includes a speaker assembly or receiver having a sound outlet opening in a first end of the receiver; and (3) a tube of resiliently flexible material having first and second ends, and a through passageway between those ends, the first end of the tube engaging the receiver with the sound outlet opening communicating with the passageway, and the second end being engaged with (e.g., pressed against or attached to) the casing at the through opening with the passageway communicating through the outer surface of the casing. The support pad includes a layer of conformable or ductile metal (e.g., aluminum) having first and second opposite major surfaces, and a layer of viscoelastic material along its first major surface. The support pad has an attachment portion with the layer of viscoelastic material along the attachment portion being adapted to be adhered to a side surface of the receiver, and has at least one tab portion adapted to project away from the receiver adjacent a second end of the receiver opposite its first end. The tab portion is adapted to be shaped so that the second major surface along the tab portion is adjacent and conforms to the inner surface of the casing and can be adhered to the inner surface of the casing with the tab portion shaped and positioned so that the first end of the receiver is against the first end of the tube and so that the attachment portion of the support pad and the receiver are spaced from the inner surface of the casing.
Such support for the receiver provides vibration isolation because the receiver does not touch the casing and is only mounted on the casing by the layer of viscoelastic material by which it is adhered to the support pad and its contact with the resiliently flexible tube through which it emits sound, and also provides vibration damping because of the vibration damping properties of that viscoelastic material.
The layer of conformable metal should be easy to bend, and should retain a shape to which it is bent. By xe2x80x9cconformable metalxe2x80x9d, we mean a metal which yields to retain a shape to which it is bent without recoil. A suitable layer of conformable metal is of aluminum having a thickness in the range of 0.002 to 0.007 inch or 0.005 to 0.018 cm, and preferably having a thickness in the range of 0.004 to 0.005 inch or 0.010 to 0.013 cm. Other suitable but more expensive conformable metals could include copper and gold.
The layer of viscoelastic material should have a thickness of at least 0.002 inch or 0.005 cm, and preferably has a thickness of at least 0.007 inch or 0.018 cm (e.g., 0.012 inch or 0.03 cm) which allows it to more easily conform to the side surface of the receiver. The layer of viscoelastic material should have the properties of the viscoelastic material described in U.S. Pat. No. 4,969,534, the content of which patent is hereby incorporated herein by reference, including that at a frequency of 1000 Hz and a temperature of 38 degrees centigrade, the viscoelastic material will produce a dynamic shear loss modulus Gxe2x80x3 of at least 0.5xc3x97107 dynes/cm2, and preferably that at a frequency of 1000 Hz and a temperature of 38 degrees centigrade, the viscoelastic material will produce a dynamic shear loss modulus Gxe2x80x3 of at least 1.5xc3x97107 dynes/cm2. Also, the viscoelastic material should be tacky as described in U.S. Pat. No. 4,969,534 to allow it to be adhered to a surface such as the surface of the receiver in the manner of a pressure sensitive adhesive. Advances in the art of viscoelastic materials now provide access to iron free viscoelastic materials which should be useful as the layer of viscoelastic material on the support pad to restrict corrosion when the support pad is used in the moist environment of a hearing aid.
The support pad can include two tab portions, each adapted to projecting away from the second end of the receiver and from a different edge of the side surface of that receiver, which tab portions are each adapted to be shaped so that the second major surface of the layer of metal along the tab portion is adjacent and conforms to the inner surface of the casing and can be adhered to that inner surface of the casing. The use of two such tab portions provides more possible locations along the inner surface of the casing at which a person assembling the hearing aid can adhere at least one of the tab portions to support the receiver.
Also, the support pad can further include a layer of polymeric material adhered over the surface of the layer of viscoelastic material opposite the layer of soft metal on its tab portion or portions. This layer of polymeric material will allow a person assembling the hearing aid to use an implement such as a tweezers to shape the tab portion so that the second major surface along the tab portion is adjacent and conforms to the inner surface of the casing without having the end of the implement stick to the layer of viscoelastic material.